Alloy steel for elevated temperature service



United States Patent 3,257,200 ALLOY STEEL FOR ELEVATED TEMPERATURE SERVICE John M. Hodge, Pleasant Hills Borough, Pa., assignor to United'States Steel Corporation, a corporation of Delaware No Drawing. Filed Dec. 10, 1962, Ser. No. 243,581

a 3 Claims. (Cl. 14836) This application relates to alloy steels having improved elevated temperature properties and more particularly to steels of the 2%% chromium-1% molybdenum type having increased elevated temperature strength.

Alloy steels of the 2%% chromium-1% molybdenum type have been widely used for elevated temperature uses involving temperatures up to about 1050 F. For such service, these steels have a unique combination of properties of good stress rupture strength, excellent room and elevated temperature not-oh toughness and good weldability and fabricata'bility. It has heretofore been proposed to increase the elevated temperature strength and stress-rupture properties of such steels by the addition of strong carbide forming elements such as vanadium, titanium and columbium'so that the steels can be used at higher temperatures. However, such attempts to improve the elevated temperature properties have resulted in a marked increase in notch sensitivity.

It is accordingly an object of this invention to provide an alloy steel of the 2%% chromium-1% molybdenum type having good strength properties at temperatures of 1l00' F. and higher along with good notch sensitivity.

The foregoing and further objects will be apparent from the following specification.

I have discovered that steel within the following composition levels has good elevated temperature properties and at the same time good notch sensitivity:

Carbon .20 max. Manganese .30/ .90 Silicon .50 max. Chromium 2.0/4.0 Molybdenum .50/2.0 Vanadium .50/1.0 Columbium 0/.15

with the balance essentially iron and residual amounts of other elements.

In a preferred embodiment, the steel is made within the following range:

Carbon .10/.15

Manganese .30/.60 Silicon .20/.40 Chromium 2.0/2.5 Molybdenum .90/ 1.10 Vanadium .50/.70

Columbium 1 .05/.l5

Steel Designa- 0 Mn P S Si Cr Mo V Cb tion Heat-treatment 1800 F., 1 hour Air-cool. 1300 F., 8 hour Air cool.

Heat-treated as 4-inch square samples.

TAB LE II.-TENSILE PROPERTIES Test Yield Tensile Elonga- Reduc- Steel Des- Tempera- Strength Strength, tion, tion of ignation ture, F. (0.2% Offp.s.i. percent Area,

set), p.s.i. percent RT 94, 500 111, 23. 0 70. 0 550 76, 800 93, 300 18. 0 71. 5 700 76, 600 93, 400 17. 0 71. 5 A 850 69, 900 84, 900 21. 0 71. 0 1, 000 67, 100 73, 100 20. 0 71. 5 1, 100 63, 800 68, 500 17. 0 75.5 1, 200 54, 200 58, 600 26.0 81. 5 1, 300 44, 500 48, 000 30. 0 83. 0 RT 94, 600 110, 600 22. 0 75. 4 550 73, 100 91, 300 19. 0 68. 5 700 74, 500 89, 800 19. 0 71. 0 B 850 72, 500 84, 600 22. 0 75. 4 1, 000 67, 300 74, 500 22. 0 79. 6 1, 100 58,900 64, 100 22. 0 81. 0 1, 200 54, 500 56, 000 20. 0 73. 9 1, 300 40, 500 44, 100 36. 0 85. 8

TABLE III.STRESS RUPTURE PROPERTIES I have further discovered that attainment of optimum notch toughness is dependent upon the use of suitable austenitizing temperatures during heat treatment. The temperature used must be high enough to dissolve a large proportion, but not all, of the vanadium carbides and, if columbium is present, to dissolve only a very small percentage of the columbium carbides. An austenitizing temperature of 1800 F. maintained for one hour produces such result. Since this is a time/temperature phenomenon, high temperatures at shorter times or lower temperatures at longer times may be used provided the foregoing objective is obtained, i.e. dissolving most but not all of the vanadium carbides while dissolving only a small amount of the columbium carbides.

From the foregoing data, it is evident that the steels of this invention possess high temperature strength, excellent stress rupture properties at temperatures as high as 1200 F. along with excellent notch toughness at 0 F. and with acceptable toughness as low as l00 F.

The results obtained are believed to result from the use of an excess of vanadium beyond the solubility limit for vanadium carbides at the austenitizing temperature used in practice. The undissolved carbides serve as grain growth inhibitors so that the steel when applied in service has a relatively fine grained microstnucture and is favor able to notch toughness. The eifect may be further augmented by small additions of columbium, which is of limited solubility at the austenitizing temperatures used in practice. The very small amount of columbium car- Patented June 21, 1966:

bide that is dissolved at the austenitizing temperature precipitates as very stable carbides that improve the elevated-temperature stability. The undissolved colu-mbium carbides serve as grain-growth inhibitors and thereby augment the grain refinement.

While I havesh-own and described several specific embodiments of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of my invention, as defined in the appended claims.

I claim:

1. Alloy steel characterized by good notch sensitivity and together with good strength properties at temperatures above 1100 F., said steel containing essentially Carbon .10/.l5 Manganese .30/.60 Silicon .20/.40 Chromium 2.0/2.5 Molybdenum .90=/1.1O Vanadium .50/.70 Columbiurn .05/.15

balance essentially iron and residual amounts of other elements. i

2. Alloy steel characterized by good not-ch sensitivity and together With good strength properties at temperatures above 1100" F., said steel containing essentially Carbon .l/. l Manganese .30/.60 Silicon .20/.4O Chromium 2.0/2.5 Molybdenum .90/ 1.10 Vanadium .50/.70- Columbium .05/.15

balance essentially iron and residual amounts of other elements, said steel having been heat treated to dissolve a substantial portion but not all of the vanadium carbides and only a small percentage of the col'umbium carbides.

3. Alloy steel characterized by good notch sensitivity and together with good strength properties at temperatures above 1100 F., said steel containing essentially Carbon .10/.15 Manganese .30/.60 Silicon 20/40 Chromium 2.0/2.5 Molybdenum .90/ 1.10 Vanadium .50/.7O Columbium .05/.l5

balance essentially iron and residual amounts of other elements,' said steel having been austenitized for about one hour at 1800 F. to dissolve a large proportion but not all of the vanadium carbides and a small percentage of the columbium carbides.

References Cited by the Examiner UNITED STATES PATENTS 2,645,574 7/1953 Clark 126 2,895,861 7/1959 Bastion 75-126X 2,968,549 1/1961 Brady 75-126 3,003,868 10/1961 Zeno etal. a 75-1 26 DAVID L. RECK, Primary Examiner.

ROGER L. CAMPBELL, Examiner.

R. O. DEAN, P. WEINSTEIN, Assistant Examiners. 

1. ALLOY STEEL CHARACTERIZED BY GOOD NOTCH SENSITIVITY AND TOGETHER WITH GOOD STRENGTH PROPERTIES AT TEMPERATURES ABOVE 1100*F., SAID STEEL CONTAINING ESSENTIALLY 